The present invention relates to the field of mass storage devices. More particularly, this invention relates to actuator arms used in such devices.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer is typically placed on a small ceramic block, also referred to as a slider that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (ABS) which includes rails and a cavity between the rails. When the disc rotates (generally, at rotational speeds of 10,000 RPM or higher), air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air-bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring, which produces a force on the slider directed toward the disc surface. The various forces on the slider equilibrate, so that the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also required to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of the disc drive. Each track on a disc surface in a disc drive is further divided into a number of short arcs called sectors. Servo feedback information is used to accurately locate the transducer head on to the tracks/sectors. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator assembly is composed of many parts that contribute to the performance required to accurately hold the read/write head in the proper position. There are two general types of actuator assemblies, a linear actuator and a rotary actuator. The rotary actuator includes a pivot assembly, an arm, a voice coil yoke assembly, and a head gimbal suspension assembly. The rotary actuator assembly pivots or rotates to reposition the transducer head over particular tracks on a disk. A suspension or load beam is part of the head gimbal suspension assembly. The rotary actuator assembly also includes a main body, which includes a shaft and bearing about which the rotary actuator assembly pivots. Attached to the main body are one or more arms. One or typically two head gimbal suspension assemblies are attached to the arm.
One end of the suspension is attached to the actuator arm assembly. The transducer head, also known as a read/write head, is found attached to the other end of the suspension. One end of the actuator arm assembly is coupled to a pivot assembly. The pivot assembly, in turn, is connected to a voice coil motor attached to a voice coil yoke on the main body of the actuator assembly. The other end of the actuator arm assembly is attached to the head gimbal suspension assembly. The head gimbal suspension assembly includes a gimbal to allow the read/write head to pitch and roll and follow the topography of the imperfect memory disc surface. The head gimbal assembly also restricts motion with respect to the radial and circumferential directions of the memory disc. The suspension assembly is coupled to the actuator arm assembly as part of the main body of the actuator assembly, which holds the pivot support and is coupled to the voice coil motor.
Fast read/write operation is critical to the performance of the hard disk. Hard disc drive being a mechanical device and actuator arms being cantilevered assemblies, which act as spring-mass-damper systems, are susceptible to vibrations at their natural frequencies. These vibrations could be excited by external forces, such as the one generated by the spindle motor or voice coil motor during positioning of the read/write head above a target track during a read/write operation. These resonant frequencies could affect the precise positioning of the head over the track, which would inevitably delay the read/write operation. As the capacity of the hard disc is increased to meet demands of increased storage requirements, the tack widths are being progressively reduced. Slightest vibration in the actuator arm could off-track the head uncomfortably close to an adjacent track during a read/write operation. This can result in track encroachment and data corruption. Every closed loop servo motor system has a predetermined bandwidth in which mechanical resonances occurring within the bandwidth degrade the performance of the servo motor system. The actuator arm is one key source of unwanted mechanical resonances. Accordingly, the bandwidths of most servo motor systems are designed so that resonance of the actuator arm occurs outside the bandwidth.
The actuator arm is driven by the voice coil motor. The voice coil motor includes moving voice coil magnet assembly to move the head over the rotating disc. The voice coil is supported by a fantail or overmould structure of the actuator arm and is hovering in a magnetic field. Generally the magnetic field produced by a single magnet is diverging with numerous stray flux compared to that of a double magnet configuration. This diverging magnetic field flux of a single magnet configuration accentuates the tipping of the actuator arm and excites the resonant frequencies of the actuator arm. During a bending resonant mode of an actuator arm carrying top and bottom head-gimbal-assemblies, the top and bottom suspension assemblies vibrate out of phase from each other. During such bending resonant mode of the top head-gimbal-assembly and the coil would be 180 degrees out-of-phase with the bottom head-gimbal-assembly and the coil. In such cases, the servo controller can only control one of the head-gimbal-assembly, while the other head-gimbal-assembly vibrates out-of-phase with the coil. This means that only one head-gimbal-assembly is controlled and stable while the other head-gimbal-assembly is vibrating. This problem gets worse when the actuator arm is reading or writing data at the innermost and outermost positions of a disc.
Actuator arms and suspensions can be made thicker to increase the bending and torsion mode frequencies, but the greater mass significantly degrades the performance of the actuator assembly by increasing the moment of inertia of the arm. Inertial increase will increase the access time for moving the transducer between data tracks. Yet another problem of increasing the arm and suspension thickness is, it can increase the current requirements necessary to move the voice coil motor. Increased current results in increased heat within the disk enclosure and increased power requirements.
What is needed is an actuator arm that produces a reduced out-of-phase motion between the top and bottom head-gimbal assemblies during an arm bending resonant mode of the actuator arm to provide a better stability to the servo motor system. Further what is needed is to reduce off-track motion of the transducer head/slider due to the out-of-phase motion in the actuator arm during seek and track follow operations of the disc drive.
Disc drives are made with an actuator designed with at least one asymmetrical arm having two masses that move out of phase. According to a preferred embodiment, the arm is modified by removing a portion of the higher-energy (longitudinal) side, the one with the greater total time-averaged strain energy. This modification reduces the phase difference between the first and the second arm in general, and particularly for a relevant frequency range of interest. Disc drives made by this method exhibit a generally reduced out-of-phase motion, enhancing performance during seeking and track following, especially for arms with two heads that interact with surfaces of opposing discs.
Additional features and benefits will be apparent upon a review of the following figures and their accompanying detailed description.